An active matrix display device creates images by writing a voltage in accordance with display data into pixels selected for each line individually and sequentially from pixels arranged in a matrix pattern. In order to select pixels for each line individually and sequentially, a shift register allowing an output signal (scanning signal) to sequentially shift depending on a clock signal is used as a gate driver. If dot sequential drive is performed, a similar shift register is formed in a source driver.
Further, in a liquid crystal display device and the like, a gate driver may be integrally formed in a production process of a thin-film transistor (TFT) in a pixel. For example, when a TFT in a pixel is made of amorphous silicon, it is preferable that a shift register functioning as a gate driver be also made of amorphous silicon for a reduction in production costs. Thus, in recent years, the formation of a gate driver on a panel, that is, gate monolithic fabrication, has been developed. The term “gate monolithic fabrication” is also associated with the terms such as “gate driver-free”, “built-in gate driver in panel”, and “gate in panel”.
A TFT made of amorphous silicon (hereinafter, also referred to as a-Si TFT) has low mobility, and therefore needs a high driving voltage. Accordingly, particularly in a large-sized display device, a high voltage needs to be supplied to a scanning signal line in order to drive an a-Si TFT in a pixel. Therefore, the channel width of the a-Si TFT in a gate driver is set to be large, and for example, it is set in mm order or cm order in the whole TFT.
As such an a-Si TFT for agate driver, an a-Si TFT including a combination of a U-shaped source electrode line and an I-shaped drain electrode line is disclosed (see, for example, Patent Literature 1).
A technology for forming a TFT for an optical sensor in a liquid crystal display device is disclosed (see, for example, Patent Literature 2).